MICROSTRUCTURED SURFACES FOR ENHANCED PHASE CHANGE HEAT

TRANSFER

TECHNICAL FIELD.

[0001] The present disclosure relates generally to microstructured material, and more particularly to a thermally conductive substrate having formed thereon a microstructured surface layer of sintered metal surfaced nanoparticles providing a hierarchical void structure for facilitating two-phase heat transfer.

BACKGROUND ART

[0002] Materials with a porous microstructure have been used as catalysts and to augment energy transfer processes in batteries, fuel cells, supercapacitors, and heat transfer devices such as heat pipes and vapor chambers. In the case of a phase change heat transfer process such as pool boiling, the use of porous or structured surfaces is the primary mechanism that can be used to enhance the heat flux or to delay the onset of a critical heat flux. See, for example, X. S. Wang, Z. B. Wang, Q. Z. Chen, "Research on Manufacturing Technology and Heat Transfer Characteristics of Sintered Porous Surface Tubes", Advanced Materials Research, Vols. 97-101 , pp. 1 161 -1 165, Mar. 2010. Several methods for the fabrication of such surfaces can be found in the literature, ranging from sintering of small particles to self-assembly or microfabrication. See, for example, Chinmay M. Patil and Satish G. Kandlikar, "Review of the Manufacturing Techniques for Porous Surfaces Used in Enhanced Pool Boiling," Heat Transfer Engineering, vol. 35 no. 10, 2014, pp. 887- 902.

SUMMARY OF THE DISCLOSURE

[0003] In accordance with one aspect, the present disclosure provides an article of manufacture including a thermally conductive substrate and a

microstructured surface layer of sintered particles on the substrate. The sintered particles have outer metal surfaces binding the sintered particles together. The sintered particles have a particle size less than 10 microns. The microstructured surface layer has regions where the particles are concentrated alternated with void regions absent of particles. The void regions have a hierarchical structure including: (i) small voids smaller than one micron; (ii) large voids larger than the small voids; and (iii) intermediate size voids having a size larger than the small voids and smaller than the large voids, the intermediate size voids being elongated and forming channels between walls of the particles.

[0004] In accordance with another aspect, the present disclosure provides a method of making a microstructured surface layer on a thermally conductive substrate. The method includes: (a) making a suspension of particles in a solvent comprising a binder material, the particles having metal surfaces and the particles having a particle size of less than 10 microns; and then (b) coating the substrate with the suspension to form a surface layer on the substrate; and then (c) cooling the coated substrate below a freezing point of the solvent to induce segregation between the particles and the frozen solvent; and then removing the frozen solvent from the surface layer; and then (e) heating the coated substrate to sinter the particles together to form the microstructured surface layer on the substrate.

[0005] In accordance with another aspect, the present disclosure provides a method of two-phase heat transfer including evaporating a liquid at an evaporator to absorb heat from the evaporator and transforming the liquid to a vapor, and condensing the vapor at a condenser to release heat to the condenser and transforming the vapor back to the liquid. The evaporator includes a microstructured surface layer of sintered particles on a thermally conductive substrate, as provided above. The condenser also includes a microstructured surface layer of sintered particles on a thermally conductive substrate, as provided above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] FIG. 1 is a pictorial diagram of a microstructured surface layer on a substrate showing a hierarchical void structure at three different length scales.

[0007] FIGS. 2 to 5 are schematic diagrams showing a series of operations in a freeze casting process. [0008] FIG. 6 is a flowchart of a method of fabricating the microstructured surface layer introduced in FIG. 1 .

[0009] FIGS. 7 to 12 are schematic diagrams showing a series of operations in a lithographic process.

[0010] FIGS. 13 to 17 are schematic diagrams showing a series of operations in a molding process.

[0011] FIG. 18 is a cross-section view of a vapor chamber spreading heat from a central processing unit (CPU) to a heat sink.

[0012] FIG.19 is a top view of a microstructured surface on a lower inner wall of the vapor chamber of FIG. 18.

[0013] FIG. 20 is a cross-section view of a vapor chamber including

microstructured surfaces of an evaporator and a condenser having free-standing parallel-spaced interdigitated fins.

[0014] FIG. 21 is a high magnification scanning electron micrograph of a microstructured surface.

[0015] FIG. 22 is a medium magnification scanning electron micrograph of the microstructured surface introduced in FIG. 21 .

[0016] FIG. 23 is a low magnification optical micrograph of a microstructured surface as formed by a molding or templating process in order to create large voids or features in this surface layer.

[0017] FIG. 24 is a graph of heat flux as a function of excess temperature for a bare copper substrate and for copper substrates having silver microstuctured surface layers of four different silver particle loadings.

[0018] FIG. 25 is a photograph of boiling on the surface of a copper substrate that does not have a microstructured surface layer.

[0019] FIG. 26 is a photograph of boiling on a silver microstructured surface layer on a copper substrate. [0020] FIG. 27 is a photograph showing the wettability that can be obtained on a strongly hydrophilic surface, which can be used on the evaporator side for evaporation and fluid transport.

[0021] FIG. 28 is a photograph showing the wettability that can be obtained on a strongly hydrophobic surface, which can be used on the condenser side for condensation and rejection of the droplets back to the condenser side.

[0022] FIG. 29 is a graph showing the evaporation rate of a water droplet from a microstructured surface according to the instant disclosure compared to the evaporation rate from a conventional copper surface.

[0023] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown in the drawings and will be described in detail. It should be understood, however, that it is not intended to limit the invention to the particular forms shown, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] The following detailed description is directed to a microstructured material especially adapted to enhance liquid to vapor phase change processes such as boiling, evaporation and condensation. The microstructured material may also be adapted to enhance the movement or transport of liquid on the

microstructured material by capillary action. The microstructured material is disposed as a surface layer on a thermally conductive substrate in order to receive heat from or deliver heat to the substrate while containing the liquid and vapor. The

microstructured surface layer can have a typical thickness of 100 microns to several mm (e.g., up to 1 , 2, 3, 4, 5 mm in thickness) on the substrate. By enhancing the phase change processes taking place on the surfaces of the microstructured material, the transfer of heat to and from these surfaces will also be enhanced compared to that from a standard surface.

[0025] The microstructured material is comprised of sintered particles on the substrate. The sintered particles have outer metal surfaces binding the sintered particles together. For example, the sintered particles are solid metal particles, such as silver particles or copper particles.

[0026] The sintered particles could also be made of a core of non-metallic material coated with metal. For example, the non-metallic material could be boron nitride or graphite. This form of construction substitutes some of the more expensive metal such as silver or copper with less expensive non-metallic material. The non- metallic material may also reduce the weight of the microstructured material. Non- metallic particles such as boron nitride or graphite also have sufficiently high values of thermal conductivity so as to facilitate efficient heat transfer.

[0027] The microstructured material may also be constructed of a mixture of metallic and non-metallic particles. The non-metallic particles can be held in the microstructured material by the metallic particles being sintered together.

[0028] The sintered particles have a particle size less than 10 microns, so that the microstructured material will have small voids smaller than one micron and the microstructured material will provide a large surface area for vapor phase change processes.

[0029] The microstructured surface layer is fabricated in such a way that it has regions where the particles are concentrated alternated with void regions absent of particles, and the void regions have a hierarchical structure including: (i) the small voids smaller than one micron; (ii) large voids larger than the small voids; and (iii) intermediate size voids having a size larger than the small voids and smaller than the large voids, the intermediate size voids being elongated and forming channels between walls of the particles. The microstructure surface layer can also be shaped by a molding process, or a lithographic or micro-machining process, to have large voids, channels, or ridges. The size, shape and orientation of these voids can be controlled during the fabrication process to enhance phase change or capillary transport.

[0030] FIG. 1 shows an example of such a hierarchical void structure spanning several length scales. The hierarchical void structure occurs within a microstructured surface layer 22 on a substrate 21 . The hierarchical void structure includes small voids 41 with typical size < 1 urn between the metallic or nonmetallic particles 42, 43 inside the regions where particles are concentrated. The small voids 41 are largely determined by the packing density of the metal particles and to some extent the degree to which these particles are sintered. These voids (the small voids) are smaller than the typical particle diameter.

[0031] The hierarchical void structure also includes intermediate sized voids 31 elongated in shape between the regions 32, 33, 34 where the particles are concentrated. These intermediate size voids 31 have a typical width of 5 - 20 urn and can be from 20 to 100 urn in length and form channels between the walls of nanoparticles. During the fabrication process these elongated medium voids 31 can be aligned so that their long axes show a preferred orientation to promote a flow of liquid from a condenser to an evaporator in a heat transfer device. The intermediate voids 31 are the result of the ice crystal templating during the freeze process. The rate of freezing has an influence (faster freeze leads to smaller ice crystals) but typically the width of the intermediate voids is in the range of about 5 microns to about 20 microns.

[0032] The hierarchical void structure also includes larger void regions 23 where no particles are present. The large voids 23 are structures that may be formed mechanically by moulding or templating during the casting process. This provides a more controlled range of dimensions. These voids can have dimensions of 100 urn to several mm (e.g., up to 1 , 2, 3, 4, 5, 10, or 15 mm in size) and can be of random shape or precisely defined shape and can be produced by microfabrication or molding operations during the fabrication. For example, as shown in FIG. 1 , the larger void regions 23 are valleys between parallel-spaced upstanding ridges 24.

[0033] The microstructured surface layer 22 has an effective surface area that is significantly larger than that of the substrate 21 on which it is disposed. The microstructured surface layer 22 can also be provided with either a hydrophilic surface or a hydrophobic surface, or both a hydrophilic surface and a hydrophobic surface. A hydrophobic surface is desirable for a microstructured surface layer of a condenser, so that the droplets of liquid formed by condensation of vapor will be more easily rejected from the condenser surface. A hydrophilic surface is desirable for a microstructured surface layer of an evaporator, so that liquid will be drawn to the surface where it will evaporate. In either case, the magnification of surface area, together with the void structure in the material and selective wettability of the surface controls the vapor-liquid processes on the surface, leading to an enhanced rate of phase change and heat transfer to or from the surfaces.

[0034] Thus, the article of manufacture including the substrate 21 and the microstructured surface layer 22 on the substrate can be a very useful component of high heat transfer devices such as heat pipes or vapor chambers such as thin vapor chamber heat spreaders used in the cooling of central processing units in thin form factor electronic devices such as tablet computers.

[0035] The microstructured surface layer 22 can be fabricated on the substrate 21 in such a way as to contain a high density of voids of a desired size and orientation, and to control the size and orientation of these voids during the fabrication process. This is an improvement over current methods of forming such surface by the conventional method of sintering of particles as it provides much greater control over the morphology of the void structure. At the same time it also represents a simpler and more flexible process than using complicated and expensive microfabrication techniques for the formation of such structured surface features. In particular, the microstructured surface layer can be fabricated on the substrate by using a freeze casting process. The freeze casting process can be combined with selective molding and microfabrication operations.

[0036] The idealized formation of a porous structure by freeze casting is illustrated in FIGS. 2 to 5. In FIG. 2, a suspension 51 of nanoparticles is subjected to a temperature gradient (indicated by the large arrow) to initiate freezing. The entire suspension 51 is not frozen at the same time, and instead the freezing in this example begins on the surface of the substrate 53 at nucleation sites 52 and then the boundary between the frozen suspension and the non-frozen suspension moves along a freeze direction aligned with the temperature gradient. In this example, the freeze direction is perpendicular to the substrate.

[0037] In FIG. 3, the growing ice crystals 54 reject the nanoparticles and any impurities. In FIG. 4, the ice is removed by sublimation in a freeze drying operation, leaving walls 55 of nanoparticles. In FIG. 5, in an anneal operation, the microstructure is heated to sinter the nanoparticles together resulting in a sintered microstructure 56 and improved mechanical strength.

[0038] FIG. 6 shows a sequence of operations for forming the microstructured surface layer on the thermally conductive substrate using such a freeze casting process. In a first operation 61 , a suspension of the desired metallic and non- metallic nanoparticles is made by suspending the nanoparticles in a solvent that contains a binder. All materials used can be commercially obtained from vendors such as Advanced Material (CT, USA) or Sigma-Aldrich. If necessary a surfactant can further be added to prevent agglomeration of the nanoparticles. The binder should be soluble in the solvent. In the case of using water as the solvent, a water soluble binder polymer such as polyvinyl alcohol or polyethylene oxide can be used. These polymers can serve as both binder and surfactant. The binder is dispersed and dissolved in the solvent after which the nanoparticles are dispersed in this solution. Mixing and dispersion of the nanoparticles can be facilitated by methods such as vortex mixing and ultrasound disruption. A typical mass loading of 1 to 10% of binder in solvent can be used, while typical mass loadings of 10 - 40% for the nanoparticles can be used. The upper limit of nanoparticles concentration is determined by the maximum amount that the solvent can maintained in suspension without particle agglomeration or sedimentation.

[0039] In a second operation 62, the thermally conductive substrate is prepared to receive a coating of the suspension. The substrate should typically be a metal with high thermal conductivity such as copper or aluminum. Prior to applying the suspension to the substrate, the substrate should be cleaned of any

contaminants like oil or grease. In order to remove any such contaminants, solvents such as acetone or ether can be used after which the solvent can be removed by washing with a solvent such as isopropanol and drying under an inert gas such as nitrogen. In the case of a substrate such as copper, the native oxide can be removed by etching in glacial acetic acid. If desired, preferred nucleation sites can be prepared on the substrate. Such preferred nucleation sites can be achieved by creating mechanical disturbances on the surface, such as by mechanical scratching or etching of the surface. [0040] Adhesion of the microstructured surface layer to the substrate can be enhanced by the deposition of a thin metal surface layer of a material such as silver on the substrate prior to the application of the suspension to the substrate. This thin metal surface layer can be deposited by methods such as vacuum evaporation, sputtering or plating. This thin metal surface layer may have a thickness in the range 10 nm to 500 nm.

[0041] In the case of using a water-based suspension, the adhesion of the suspension to the substrate can be enhanced by the creation of a hydrophilic surface on the substrate. This can be done by the deposition of a compound such as 16- mercaptohexadecanoic acid on the surface. This compound can be deposited from an ethanol solution onto the metallic substrate surface during a self-assembly process.

[0042] In a third operation 63, the substrate is coated with the suspension to form a surface layer on the substrate. This coating operation may include lithography or molding to form free-standing structures, as further described below with reference to FIGS. 7 to 12. The suspension can be coated onto the substrate by methods such as dip coating, drop coating, spraying or painting, producing the desired thickness and spread over the substrate.

[0043] In a fourth operation 64, the solvent can now be solidified by lowering the temperature below the freezing point of the solvent. This leads to freezing of the solvent and also induces segregation between the solvent and the nanoparticles plus binder due to impurity rejection by the freeze front of the solidifying solvent. This freezing process can be initiated by cooling of the substrate such as on a cold finger, but it can also be initiated by placing the coated substrate in a cold fluid environment. In the case of a cold finger the temperature can be controlled by methods such the circulation of a cold fluid or by placing the cold finger in contact with a thermoelectric Peltier cooler. In each case the rate of cooling can be controlled so as to control the rate of freezing and so control the size of solvent crystals formed.

[0044] In a fifth operation 65, the solvent crystals are removed by sublimation in a freeze drying process. This removes the solvent phase, revealing the

nanoparticles and binder as was templated by the freezing solvent crystals. The removal of the solvent crystals then leaves a "green structure" of particles held together by the polymer binder. This "green structure" will contain voids as templated by the frozen solvent. In this context "green" indicates not a green color but a structure capable of maintaining a degree of structural integrity up to and during an annealing operation. In general, the locations of the nanoparticles in the annealed structure will be substantially the same as the locations of the nanoparticles in the green structure, although the annealed structure may be reduced in size compared to the "green structure" due to some shrinkage during the annealing operation.

[0045] In a sixth operation 66, the green structure is heated in an annealing operation to sinter the nanoparticles together to form the microstructured surface layer on the substrate. For example, the green structure is heated under an inert gas flow or in vacuum in order to sinter the nanoparticles and so provide mechanical strength to the structure. At the same time the heating of the annealing operation evaporates the binder polymer. The precise temperature and duration of this annealing operation should be dependent upon the type of particles and the binder used as well as the thickness of the material layer. In the case of a 1 mm thick layer of silver nanoparticles with a polyvinyl alcohol binder an anneal under oxygen free Argon gas at a temperature of 350 °C for a time of approximately three hours will achieve the desired annealed microstructured surface layer.

[0046] In a seventh operation 67, the annealed microstructured surface layer may undergo further processing, such as creating a hydrophilic surface, a

hydrophobic surface, or both a hydrophilic surface and a hydrophobic surface.

[0047] The size, shape and orientation of the different voids present in the material can be controlled by process and material parameters such as nanoparticle size, mass loading of the particles, concentration and type of the binder, and freeze rate. In general the freeze rate should be kept low enough to ensure segregation between the solvent freeze front and the nanoparticle and binder component.

[0048] The size of small voids between individual nanoparticles is controlled by the size of the particles and the freeze rate, with smaller particles and a lower freeze rate leading to smaller voids. [0049] The size and shape of the elongated medium voids is largely determined by the freeze rate but also to a smaller degree by the particle size and by the nature of the binder used. A slow freeze rate will lead to larger voids, while a very fast freeze rate will decrease the size of these voids and also regular elongated shape.

[0050] The orientation of the elongated voids can be controlled to produce voids with a high degree of alignment of their long axes. This can be achieved by the creation of preferred nucleation sites on the substrate. Such preferred nucleation sites can be achieved by creating mechanical disturbances on the surface, such as by mechanical scratching or etching of the surface. These disturbances should be aligned in the direction desired for the preferred orientation of the voids and should have close to the spacing that the solvent crystals will show at the freeze rate employed.

[0051] One method to form such a surface is by photolithography and etching as shown in FIGS. 7 to 12. In FIG. 7, a substrate 71 is cleaned in order to receive a photo-resist coating 72 as shown in FIG. 8. In FIG. 9, a mask 73 is laid over the photo-resist coating 72 and the mask is illuminated by a light source 74 so that selected areas of the photo-resist are illuminated through holes 75 in the mask 73. In FIG. 10, the mask has been removed and the photo-resist has been developed and the regions of the photo-resist that were illuminated have been washed away. In FIG. 1 1 , etchant has been applied over the developed photo-resist so that areas 72 of the substrate where the photo-resist has been washed away have been exposed to the etchant and have been etched away by the etchant. Finally, in FIG. 12, the photo-resist layer 72 has been stripped off of the substrate 71 .

[0052] The use of a standard photoresist and mask technique can be used to produce photoresist features 77 with the desired size and spacing on the substrate 71 . The substrate can then the etched through the open channels 76 in the pattern 72 using an etchant such as ferric chloride for a copper substrate, leaving a series of aligned channels in the substrate. These channels will act as preferred nucleation sites to the freezing solvent and result in alignment of elongated medium voids where needed in the structure. Other methods that can be used to produce the same effect of mechanical disturbances on the substrate are mechanical abrasion or laser ablation.

[0053] Large voids and features with a length scale of typically 0.1 to several mm in size (e.g., up to 1 , 2, 3, 4, 5, 10, or 15 mm in size) can be created in the material when desired by using photolithographic patterning or molding during the suspension deposition operation. This can be achieved by shaping the suspension on the substrate surface by means of a mold. This mold should be made from a material that does not dissolve in the same solvent as used for the suspension. For example, if a water-based suspension is used then the mold should be constructed from a material such as acrylonitrile butadiene styrene (ABS) which is not soluble in water but is soluble in acetone. Such a mold is used as a sacrificial mold and can be printed on a commercial 3D printer which has a printing resolution good enough to provide the dimensions needed.

[0054] FIGS. 13 to 17 show such a molding process for obtaining large voids and structures in the material by using a sacrificial mold 82. As shown in FIG. 13, the sacrificial mold 82 is placed over a substrate 81 before the application of the suspension. The sacrificial mold 82 has regions 83 for displacing the suspension. As shown in FIG. 14, the suspension 84 is cast into this mold 82 which is in contact with the substrate surface, after which the process operations of freezing and freeze drying are followed, resulting in the green structure 85 in the mold as shown in FIG. 15. When the solvent component has now been removed by the sublimation of the freeze drying operation, the mold 82 is removed by immersing in a solvent that will dissolve the mold, such as acetone for ABS. As shown in FIG. 16, removal of the mold leaves the freeze-dried green structure 85 as shaped by the mold on the substrate 81 , after which this green structure is annealed to remove the binder and sinter of the particles as described above with reference to operation 65 in FIG. 6.

[0055] Alternatively thick layers of photoresist can be used to define where the suspension contacts the substrate and pattern the deposited suspension. As with a sacrificial mold, the photoresist can then be removed after the freeze drying operation and then the remaining material is annealed. [0056] The wettability of the surface and the subsequent interaction with a liquid like water can further be controlled by the addition of hydrophilic or

hydrophobic surface layers on the surface of the material. This can be done by the coating of a compound such as 16-mercaptohexadecanoic acid or 1 - hexadecanethiol from an ethanol solution to achieve a hydrophilic or hydrophobic surface respectively. A hydrophobic surface on silver can also be obtained by annealing the material in an oxygen containing atmosphere at temperatures > 100 °C in order to form hydrophobic oxides on the surface of the silver. The photograph in FIG. 27 shows the wettability that can be obtained on a strongly hydrophilic surface, which can be used on the evaporator side for evaporation and fluid transport. The photograph in FIG. 28 shows the wettability that can be obtained on a strongly hydrophobic surface, which can be used on the condenser side for condensation and rejection of the droplets back to the condenser side. A graph showing the evaporation rate of a water droplet from a microstructured surface according to the instant disclosure compared to the evaporation rate from a conventional copper surface is provided in FIG. 29. The graph shows that the droplet (volume ~ 25 μΙ) evaporated in ~ 1 .2 seconds from a hydrophilic

microstructured silver surface prepared as described herein. A similar drop placed on a standard machined copper surface required ~ 22 seconds to evaporate. Thus, a greater than 10 fold increase (> 10 x) in the rate of heat transfer from the microstructured surface was observed.

[0057] The article of manufacture comprised of the substrate and the microstructured surface layer on the substrate can be used in the manufacture of two-phase heat transfer devices such as vapor chambers and heat pipes. FIG. 18 shows a typical two-phase vapor chamber heat spreader 91 . This device 91 has a first side or zone 94 where liquid to vapor phase changes take place (the

evaporation side or hot side) and a second side or zone 95 where vapor to liquid phase changes take place (the condensation side or the cold side). In the areas where liquid to vapor phase changes takes place, heat will be effectively absorbed by the device 91 , while in the areas where vapor to liquid phase change takes place heat will be effectively released by the device. If the evaporation side 94 is then attached to a source of heat 92 and the condensation side 95 is attached to a sink of heat 93, then heat will flow with very low resistance from the evaporation section to the condensation section and draw heat away from the heat source and allow this source to be efficiently cooled. The source of heat may be an electronic component such as a central processing unit (CPU) 92 or graphics processing unit, while the sink of heat 93 may be the enclosure of the electronic system or it may be a further cooling system such as a finned heat sink and fan.

[0058] The interior of the vapor chamber 96 can be partially evacuated of air and then filled with a working liquid which is evaporated and condensed in the different zones. The liquid that condenses in the condensation section should be returned to the evaporation section to complete the fluid cycle. The return path can be enabled by the gravity return of drops from the condensation cycle or by the capillary transport of the condensed liquid on the inside surface of the vapor chamber or by a combination of these two mechanism.

[0059] The microstructured surface layer 101 is deposited on the inside wall of such a vapor chamber 96 in order to facilitate the evaporation, condensation, and capillary transport of fluid. In the evaporation section 94 the void structure 103 of the microstructured surface layer 101 can be aligned, as shown in FIG. 19, by the methods described above to facilitate capillary movement of the liquid from any part of the section to the hot zone above the CPU 92. The surface of the material in this section can be made hydrophilic by methods such as described above to ensure efficient capillary action as well as evaporation. In the condensation zone 95 the material can be made hydrophobic by methods such as described above in order to facilitate the rejection of condensed drops from the condensation zone and their return by gravity to the evaporation section 94.

[0060] The efficacy of both the evaporation and condensation sections can be further enhanced by shaping of the deposited material to contain structures such as fins that will enhance the surface area. These structures can be fabricated by the use of microfabrication or molding operations such as described above. This can be done on either the evaporation side or on the condensation side or on both. For example, FIG. 20 shows such a finned structure on both the evaporation side 1 12 and the condensation side 1 1 1 of a vapor chamber 1 10, resulting in an interdigitated finned structure. The interdigitation of the evaporator fins with the condenser fins provides close proximity between large surface areas of the evaporator and condenser to facilitate vapor exchange between the evaporator and the condenser.

[0061] As the described microstructured material can be deposited as very thin films with a high surface area and efficient evaporation and condensation surfaces, the vapor chambers constructed from these layers can potentially be made very thin with a very low thermal resistance. Such a vapor chamber heat spreader structure would be ideal for the cooling of electronic components in space

constrained enclosures such as tablet computers.

[0062] Following is a specific example of fabricating microstructured silver surface layers on copper substrates using the freeze casting method described above, and the measurement of their enhancement of two-phase heat transfer in pool boiling. Silver nanoparticles with diameters between 100— 500 nm were used as the starting material. Suspensions were made by slowly dispersing the

nanoparticles in a 1 % solution of polyvinyl alcohol in water, producing mass loadings of nanoparticles from 10% to 40%. Vortex mixing and ultrasonic disruption were used to prevent particle aggregation and promote dispersion.

[0063] Substrates in the form of copper discs with a typical thickness of 2.5 mm and a diameter of 25 mm were machined, degreased in acetone and

isopropanol and given a final etch in acetic acid to remove surface oxides. A measured volume of the silver nanoparticle solution was then dispensed on the substrate and the substrate placed on top of a copper cold finger. A small hole in the base of the substrate allowed access to a thermocouple for temperature

measurement during the freeze casting as well as later during heat transfer tests. The temperature of the cold finger was controlled by Peltier thermoelectric elements and a controlled ramp from room temperature to -40 °C was induced in the cold finger. At the conclusion of the freeze casting, samples were removed from the cold finger and placed in a vacuum freeze drier for a period of ~ 48 hours, after which samples were annealed in a tube furnace under inert gas flow for three hours at a temperature of 350 °C.

[0064] The efficacy of the films in enhancing heat transfer during the nucleate phase of pool boiling was investigated by mounting the samples on the top of a copper heat flux finger that was heated from the bottom by cartridge heaters. The sample was then fitted into the bottom of a glass boiling vessel containing saturated, pre-boiled deionized water at atmospheric pressure as the working liquid. By measuring the temperature profile along the heat flux finger as well as the surface temperature of the copper substrate, the heat flux as a function of excess

temperature ΔΤ _βχ (ΔΤ _βχ = T _SU rface - T _sa turation) was calculated for values of ΔΤ _βχ in the range 0 - 10°C. A camera was fitted at the top of the test system enabling viewing of the sample surface during the process.

[0065] The detailed microstructure of the silver surface after annealing is shown at different length scales in scanning electron micrographs of FIGS. 21 and 22. The microstructure increased in density from the outside perimeter of the substrate to the center, i.e. in the direction of the freeze front. This is consistent with particles and other impurities being ejected by the advancing ice front and eventually concentrating in the center of the sample. This effect is least noticeable in samples with high Ag particle mass loadings or when very high freeze rates were used and may be due to the fact that in these cases the moving freeze front was unable to reject particles, leading to a more even structure over the substrate. The observed pores are broadly divided into three types based on their shape and size.

[0066] Shown in FIG. 21 are the small voids in the structure. These are micro voids formed within the nanoparticle walls, as these walls are not densely packed. These pores have typical dimensions of 0.5— 5 pm, depending on the size of the particles used.

[0067] Shown in FIG. 22 are intermediate sized voids. These are elongated porous channels separated by nanoparticle walls, with channels typically 40 to 80 pm in length and approximately 10 pm wide. The walls consisted of sintered nanoparticles that were ejected by the ice crystals and have a typical thickness of 5 to 10 pm. These channels and walls were found to be aligned over small areas (0.2 to 0.5 mm ² ) of the sample.

[0068] Shown in FIG. 23 is a structure of large voids as can be obtained by a molding or templating process. In this case a mold and template process was used to form parallel channels of approximately 5 mm wide in the microstructured surface layer.

[0069] The observed void structure of small and intermediate voids were found to be weakly dependent on the freeze rate, with higher freeze rates leading to a decrease in the size of the elongated intermediate voids. This is to be expected, as high freeze rates will produce smaller ice crystals with fast moving fronts which will destroy the periodic nature of the intermediate voids. It can also be assumed that the tool marks on the substrate act as nucleation points for ice platelets and as such it will be difficult to produce structures with better alignment of the walls and channels without more elaborate surface preparation.

[0070] FIG. 24 shows measurements of heat flow during pool boiling on the porous silver surface compared to that from on a standard copper substrate with no surface layer. Very little difference in the heat flux from the different samples were noted at low values of excess temperatures (ΔΤ _βχ < 2 °C). For the bare copper substrate, the heat flux increased relatively slowly until ΔΤ _βχ ~ 5.5 °C, after which a sharp increase in the heat flux was observed. In the case of a microporous layer present on the surface, this departure from the initial low value of heat flux occurred at much lower values of ΔΤ _βχ ranging from approximately ΔΤ _βχ = 2.5 °C for a 10% Ag mass loading to ΔΤ _βχ = 1.8 °C for a 40% Ag mass loading. The performance of samples with a mass loading of 10 ~ 30 % Ag were comparable, but the 40% mass loading had significantly better heat transfer. At an excess temperature of ΔΤ _βχ = 8 °C the first three samples produced a heat flux that was approximately 3.3 times that of the copper substrate, while the 40% silver sample had a heat flux 5.3 times that of the copper.

[0071] The formation of bubbles and their departure was found to be significantly different on the two types of surfaces at these low values of excess temperature. FIG. 25 shows a mean bubble diameter of 0.41 ± 0.16 mm for bubbles that formed on the machined copper surface at ΔΤ _βχ of ~ 2 °C. FIG. 26 shows a mean bubble diameter of 0.28 ± 0.08 mm at similar temperatures on the porous silver surfaces. The density of bubbles was also significantly higher on porous surfaces, with a typical density of 24 bubbles/cm ² on the copper surface compared to approximately 80 bubbles/cm ² on the porous surfaces. In the latter case, low contrast between bubbles and surface makes clear identification harder and together with the smaller size of the bubbles it may well lead to an underestimation of bubble density. Significant bubble departure from the microstructured surface also occurs well in advance from that on the copper surface, in agreement with the point of increase in the heat flux observed for the different samples.

[0072] FIG. 27 is a photograph showing the wettability that can be obtained on a strongly hydrophilic surface, which can be used on the evaporator side for

evaporation and fluid transport. FIG. 28 is a photograph showing the wettability that can be obtained on a strongly hydrophobic surface, which can be used on the condenser side for condensation and rejection of the droplets back to the condenser side. FIG. 29 is a graph showing the evaporation rate of a water droplet from a microstructured surface according to the instant disclosure compared to the evaporation rate from a conventional copper surface. The graph shows that the droplet (volume ~ 25 μΙ) evaporated in ~ 1 .2 seconds from a hydrophilic

microstructured silver surface prepared as described herein. A similar drop placed on a standard machined copper surface required ~ 22 seconds to evaporate. Thus, a greater than 10 fold increase (> 10 x) in the rate of heat transfer from the

microstructured surface was observed.

[0073] The different types of voids or pores in the microstructured surface layer may potentially each play a role in contributing to the enhanced heat flux. Both the small and intermediates voids will enhance the number of potential nucleation sites. Due to their channel shape with a high aspect ratio, the intermediate elongated voids may also contribute to an upward squirt or jet effect during bubble departure. This has the potential to further enhance fluid convection during the release of bubbles. The small micro voids in the walls may play a role in feeding liquid to the growing bubbles and sustain the process, while at the same time the high thermal conductivity of the silver nanoparticles as well as the associated high surface area of the porous structure will be effective in transferring heat from the copper surface into the fluid.